1) Field of the Invention
The present invention relates to an optical switch device and an optical switching method for switching optical signals.
2) Description of the Related Art
With the rapid expansion of the use of the Internet and the increase in the number of massive pieces of content, demands for communication networks having greater capacity and flexibility and realizing higher transmission speed have been increasing. In order to construct such communication networks, the optical communication technology is essential, and development and research of the optical communication are being performed in various fields.
In particular, optical switches which perform on-off control of optical signals will be important constituents of photonic networks in future. In recent years, the ultrafast optical switching technology in which nonlinear optical effects in optical fibers are positively used has been receiving attention. The nonlinear optical effects are phenomena in which the properties of glass vary with the optical intensity of light which propagates through the glass, and the linearity of the optical response is lost when the power of the light is relatively strong.
Typical optical switches which are conventionally used are the MEMS (micro-electro-mechanical system) and the waveguide switch. The MEMS is manufactured by the micromachine technology, and performs on-off control of optical signals by changing the direction of the optical path with a micron-sized mirror or a shutter. On the other hand, the waveguide switch performs on-off control of optical signals by applying heat or an electric field to an optical waveguide so as to cause a thermo-optic effect or an electro-optic effect and change the refraction index of the optical waveguide.
However, the light-collection efficiency of the MEMS is lowered due to the alignment tolerance, i.e., the tolerance of the alignment of the elements such as mirrors with an optical axis. On the other hand, in the waveguide switch, the insertion loss (the level loss occurring when light is inserted into the waveguide switch) occurs, and the extinction ratio (the ratio of the maximum to the minimum of the intensity of transmitted light) cannot be increased. Therefore, in the waveguide switch, the signal level is likely to be lowered, and noise is likely to occur.
As described above, in the conventional optical switches such as the MEMS and the waveguide switch, the signal quality deteriorates. In addition, the control of the optical switches based on electronic signal processing cannot realize high-speed switching.
On the other hand, the optical switches which utilize physical properties of light and nonlinear optical effects of optical fibers realize high-speed switching by inputting signal light and control light (excitation light) into a highly nonlinear fiber (HNLF), in which a nonlinear optical effect occurs with high efficiency, and causing parametric amplification of nonlinear variations of the refraction index in the HNLF. The parametric amplification is a nonlinear optical effect which amplifies the intensity of signal light, and a phenomenon which is caused by interaction between the signal light and the control light in the HNLF without use of a linear amplification medium such as the conventionally used EDF (erbium-doped fiber). A simple definition of the parametric amplification or the parametric oscillation is a process of generating light waves having two wavelengths ω1 and ω2 in a nonlinear optical medium by using a higher frequency ω3.
The parametric amplification in the HNLF is an optical and physical phenomenon occurring very quickly. Therefore, the response of the parametric amplification is far quicker than the mechanical switching of the optical path or the responses of other phenomena in which the refraction index of a medium is changed by a thermo-optic effect or an electro-optic effect, so that it is possible to realize high-speed switching at a speed corresponding to the input rate of ultrafast optical pulses into the HNLF, and extract each optical pulse of interest from among the ultrafast optical pulses. At this time, the optical pulses are amplified before being outputted.
As described above, the optical switches using the highly nonlinear fiber (HNLF) have the function of the high-speed switching and the function of optical amplification. Therefore, such optical switches exhibit high switching efficiency, and can realize high-quality optical switching and transmission in which noise is extremely low and deterioration of the SNR (signal-to-noise ratio) is little. Thus, there is great expectation that the above optical switching by use of the highly nonlinear fiber (HNLF) becomes a technique for realizing ultrafast optical-signal processing in the next generation.
A conventional technique using the parametric amplification is disclosed in Japanese Unexamined Patent Publication No. 2002-90788, paragraph Nos. 0013 to 0020 and FIG. 1. According to this technique, a parametric amplifier using an optical fiber and having amplification characteristics which are independent of the polarization of an inputted optical signal is used.
In the case where a desired optical pulse extracted from time-division-multiplexed (TDM) signal light is switched and outputted from an optical switch which uses parametric amplification occurring in a highly nonlinear fiber (HNLF), an optical control pulse is inputted into the HNLF in such a manner that the optical control pulse is in phase with the optical pulse to be switched, and the polarization direction of the optical control pulse makes a certain angle with the polarization direction of the optical pulse.
FIG. 20 is a diagram illustrating an outline of operations of optical switching by use of the parametric amplification. Signal light and control light are inputted into a highly nonlinear fiber F, where the signal light is a time-division multiplexed (TDM) optical signal in which optical pulses of the signal light in the channels ch1 to ch4 are time-division multiplexed.
In the case where only the optical pulses of the signal light in the channels ch1 and ch3 are to be extracted, switched, and outputted, optical control pulses p1 and p3 are inputted into the highly nonlinear fiber F in such a manner that the optical control pulses p1 and p3 are in phase with the optical pulses of the signal light in the channels ch1 and ch3, respectively, and the polarization directions of the optical control pulses p1 and p3 make a certain angle with the polarization directions of the optical pulses of the signal light in the channels ch1 and ch3, respectively. Then, the power of the optical control pulses p1 and p3 is respectively transferred to the optical pulses of the signal light in the channels ch1 and ch3 by the parametric amplification occurring in the highly nonlinear fiber F, so that only the optical pulses of the signal light in the channels ch1 and ch3 are amplified and outputted from the highly nonlinear fiber F. Although not shown, in practice, unnecessary optical signals are cut off by using a polarizer, a band-pass filter, and the like which is arranged at the output stage of the highly nonlinear fiber F.
As explained above, it is possible to perform switching by generating an optical control pulse for each of a plurality of channels time-division multiplexed in signal light, and inputting one or more of the optical control pulses corresponding to one or more of optical pulses to be switched, in the above-mentioned manner.
However, the optical pulses time-division multiplexed in the respective channels may be transmitted to the optical switch from various nodes distributed over a network. Therefore, in practice, the optical levels in the different channels may be different.
Conventionally, optical control pulses having identical levels which are preset are inputted into the highly nonlinear fiber F for switching of optical pulses of the signal light in a plurality of channels constituting signal light and having different levels. Therefore, the levels of the optical pulses after the switching become different, so that it is necessary to compensate for the differences among the levels.
FIG. 21 is a diagram provided for explaining the problems in the conventional optical switching. In FIG. 21, the signal light is a time-division multiplexed (TDM) optical signal in which optical pulses in the channels ch1 to ch4 are time-division multiplexed, the levels of the optical signals in the channels ch1 to ch4 are different. However, the levels of optical control pulses for the optical signals in the channels ch1 to ch4 are identical.
In the case where the levels of all the optical control pulses p1 to p4 for the different channels of the signal light are fixed, when the optical control pulses p1 to p4 are inputted into the highly nonlinear fiber F in the aforementioned manner, the relative level differences among the optical signals in the channels ch1 to ch4 at the output stage of the highly nonlinear fiber F correspond to the relative level differences among the optical signals in the channels ch1 to ch4 at the input stage of the highly nonlinear fiber F. That is, the switch-output levels of the optical signals in the channels ch1 to ch4 become different. Therefore, it is necessary to compensate for the differences among the optical levels at each receiver node, so that the convenience in system operation is deteriorated. In addition, in order to compensate for the differences among the levels of the optical signals in the channels ch1 to ch4, it is possible to arrange an optical amplifier for each channel, and bring the levels of the optical signals in each channel to a constant level by amplification at a stage following the highly nonlinear fiber F.
However, with the rapid increase in the communication traffic in recent years, the numbers of channels to be used have been increasing. In addition, the numbers of channels of time-division multiplexed optical pulses of the signal light in the currently used systems are very large. If, in such circumstances, the above optical amplifier for compensation for the differences among the optical levels is provided for every channel at each node in a network in which optical switching is performed, the equipment size and cost increase, and the increase in the equipment size and cost greatly impedes economical construction of an ultrafast optical communication network.